1. Field of the Invention
The subject matter of the invention relates to electric power steering systems and methods for vehicles.
2. Description of the Related Art
Power steering systems for vehicles, such as automobiles, have long been used to assist an operator (i.e., a driver) of the vehicle in turning the wheels of the vehicle. These systems traditionally have utilized hydraulic fluid to assist in movement of a steering rack. Such hydraulic power steering systems frequently utilize a fluid pump. The fluid pump may be connected to the engine of the vehicle with a belt or connected to an electric motor. The fluid pump may constantly provide fluid flow to a torque-dependent restricting orifice bridge and a hydraulic actuator, which provides the steering assist. Due to continuous flow at non-zero pressure in these systems, commonly referred to as open center hydraulic systems, the engine expends energy to operate the fluid pump whenever the engine is running. This is a wasteful use of fuel as power assist of the steering system is not constantly needed.
Modern electric power steering systems 110, as shown in FIG. 1, utilize an electric actuator 132 instead of the hydraulic actuator to provide the assist to the steering column 118 (e.g., column assisted electric systems) or rack 122 (e.g., rack assisted electric systems). An example of a column assisted electric system 110 is illustrated in FIG. 1, and includes the steering column 118 and rack 122 coupling a steering wheel 112 to vehicle wheels 114. The electric actuator 132 is operatively connected to the steering column 118 to provide the assist. Accordingly, no constantly running hydraulic fluid pump is needed, and assist is provided only as demanded. As a result, energy is conserved and fuel efficiency of the vehicle is increased.
Unfortunately, electric power steering systems often suffer from driver perception issues. For instance, the amount of effort exerted by the driver may not psychophysically coincide with the actual turning of the wheels of the vehicle and the expectations for resisting effort or perceived vehicle responsiveness. As such, the steering may feel “too loose”, “too rigid”, “sluggish”, too high in required effort, and/or too low in required effort to the driver. The relationship between road wheel movement and effort by the operator of the vehicle in such prior art systems is illustrated in FIG. 2.
Accordingly, it is desirable to provide an electric power steering system that achieves better correlation between driver effort, vehicle response, and expectations for effort and response based on subjective impressions of performance. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.